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An Introduction to the Atmosphere

The atmosphere is a mixture of different gases, particles and aerosols collectively known as air which envelops the Earth. The atmosphere provides various functions, not least the ability to sustain life. The atmosphere protects us by filtering out deadly cosmic rays, powerful ultraviolet (UV) radiation from the Sun, and even meteors on collision course with Earth. Although traces of atmospheric gases have been detected well out into space, 99% of the mass of the atmosphere lies below about 25 to 30km altitude, whilst 50% is concentrated in the lowest 5km (less than the height of Mount Everest).

Air remains remarkably uniform in composition, and is the result of efficient recycling processes and turbulent mixing in the atmosphere. Such recycling and mixing of the air helps to minimise the amount of time which man-made pollution spends in the atmosphere at any single location, thereby reducing the environmental impacts. The two most abundant gases are nitrogen (78% by volume) and oxygen (21% by volume), and together they make up over 99% of the lower atmosphere. There is no evidence that the relative levels of these two gases are changing significantly over time. In addition to nitrogen and oxygen, air contains a number of trace gases, including the noble gases argon, neon, helium, krypton and xenon, the greenhouse gases and ozone.

Despite their relative scarcity, the so-called greenhouse gases play an important role in the regulation of the Earth's climate. The natural greenhouse gases include carbon dioxide, methane, nitrous oxide and water vapour. Although ozone is also a greenhouse gas it is more commonly associated with the ozone hole and ozone depletion. By trapping heat trying to escape from the surface of the Earth to space, the greenhouse gases warm the atmosphere. Consequently the Earth surface is 33�C warmer than it would be without an atmosphere. This heating process is called the natural greenhouse effect.

Although air is well-mixed throughout the atmosphere, the atmosphere itself is not physically uniform but has significant variations in temperature and pressure with altitude, which define a number of atmospheric layers, including the troposphere, stratosphere, and mesosphere. Beyond about 50 miles (80 kilometres) altitude, the air is very very thin indeed. Layers used to describe the outer reaches of the atmosphere include the thermosphere, the ionosphere, the exosphere and the magnetosphere. Another well-known layer is the ozone layer, residing in the stratosphere and protecting life below from the harmful effects of ultraviolet (UV) radiation from the Sun. Every year, ozone holes form in ozone layer above Antarctica and the Arctic.

Most of the world�s weather systems and their related features, including clouds and rain, develop in the lowest layer of the atmosphere, the troposphere. Such weather systems, or patterns of air movement, develop as a result of the flow of heat from warmer regions of the Earth near the equator to colder regions nearer the poles. The air, and the heat it carries however, does not flow in a straight line, because of the Earth's rotation. As a consequence, the Coriolis Force deflects the air, forming patterns of air circulation, similar to circulating water in a draining sink.

During the last 200 years, mankind has begun to significantly alter the composition of the atmosphere through pollution. Although air is still made up mostly of oxygen and nitrogen, some of the levels of trace gases have been increasing, in particular the concentrations of greenhouse gases, which may be causing global warming. Some air pollutants now present in the atmosphere are completely new, such as the CFCs, which are solely man-made.